Copolymers of tetrafluoroethylene (TFE) and a co-monomer according to the formula: FSO2—CF2—CF2—O—CF(CF3)—CF2—O—CF═CF2 are known and sold in sulfonic acid form, i.e., with the FSO2— end group hydrolyzed to HSO3—, under the trade name Nafion® by DuPont Chemical Company, Wilmington, Del. Nafion® is frequently used to make polymer electrolyte membranes for use in fuel cells.
U.S. Pat. Nos. 4,358,545 and 4,417,969 disclose polymers and ion exchange membranes thereof having a hydration product of less than about 22,000 and equivalent weight of 800-1500, which have a substantially fluorinated backbone and pendant groups according to the formula: YSO3—(CFRf)a(CFR′f)b—O-backbone, where Y is hydrogen or an alkali metal, Rf and R′f are halogen or substantially fluorinated alkyl groups, a is 0-3, b is 0-3, and a+b is at least 1.
Kokai Patent Application No. SHO 58-93728 discloses at Example 3 the copolymerization of TFE and FSO2—CF2—CF2—CF2—CF2—O—CF═CF2. The resulting polymer is extrusion molded to produce a film with a thickness of 150 microns and hydrolyzed, the resulting membrane having an equivalent weight of 990. This film was further treated so as to convert sulfonic acid groups to carboxylic acid groups in a thin surface layer of one side of the membrane.